Percolation segregation of binary mixtures under periodic movement

Powder Technology 195 (2009) 73–82

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Percolation segregation of binary mixtures under periodic movement

Anjani K. Jha a,⁎, Virendra M. Puri b

a Materials Research Institute, Agricultural and Biological Engineering Department, 249 Agricultural Engineering Building, Pennsylvania State University, University Park, PA 16803, United Statesb Materials Research Institute, Agricultural and Biological Engineering Department, 229 Agricultural Engineering Building, Pennsylvania State University, United States

⁎ Corresponding author. Tel.: +1 814 441 9835; fax: +E-mail address: [email protected] (A.K. Jha).

0032-5910/$ – see front matter. Published by Elsevierdoi:10.1016/j.powtec.2009.04.013

a b s t r a c t
a r t i c l e i n f o
Article history:Received 14 December 2007Received in revised form 17 March 2009Accepted 25 April 2009Available online 7 May 2009

Keywords:Primary segregation shear cell (PSSC-II)Size ratioCoarse particleFine particlesStrain ratePotashUreaSegregation rateNormalized segregation rate

Three strain rates of 1.0, 0.5, and 0.25 Hz were selected for studying percolation segregation in binarymixtures of urea (spherical) and potash (angular). Mixed binary samples prepared from three mean coarsesizes with their corresponding three and two fines sizes for potash and urea, respectively. Herein, threecoarse mean sizes 3675, 3075, and 2580 µm and three mean fine sizes 2180, 1850, and 1550 µmwere selectedfor tests. Percolation segregation in mixed binary sample was quantified using the primary segregation shearcell (PSSC-II). Based on experimental results, the segregated fines mass, normalized segregation rate (NSR)and segregation rate of fines for binary mixtures increased with increasing strain rate from 0.25 Hz to 1.0 Hz.The NSR decreased with decreasing strain rate from 1.0 HzN0.5 HzN0.25 Hz for size ratios 1.7, 2.0, and 2.4(pb0.05). At these three strain rates, for size ratio 2.0, the NSR of coarse size 3675 µmwith fines size 1850 µmwas smaller than the NSR of coarse size 3075 µm with fine size 1550 µm in the binary mixtures (pb0.05). Atthree strain rates of 1.0, 0.5, and 0.25 Hz, the NSR for potash was higher (53%, 56%, and 46%) than the NSR forurea for the same size ratio (pb0.05).

Published by Elsevier B.V.

1. Introduction

Segregation is an unwanted phenomenon in particulates thataffects the quality of mixtures during unit operations, such as mixing,conveying, filling, discharging, and compaction. Segregation can bedefined as a bulk solid composed of particulates with differingconstituent properties that evolve to a spatially non-uniform state [1].The importance of mitigating segregation can be gauged by thediverse group of industries that are impacted; for instance, agricul-tural, ceramic, construction, food, nutraceutical, metal powder andmetallurgy, and pharmaceutical. Researchers have reported that thephysical properties of particles, energy input [2], particle movementdirection [3,4], and devices used [5–7] during particulate materialprocessing contribute towards particle segregation. Based on theabove-mentioned parameters and new findings [8–10], a re-categor-ization of the segregation mechanisms (trajectory, air current, rolling,sieving, impact, embedding, angle of repose, push-way, displacement,percolation, fluidization, agglomeration, and concentration drivendisplacement) has been proposed. Of the above-mentioned thirteensegregation mechanisms, percolation segregation is the most com-mon phenomena during unit-operations such as filling, conveying,storage, mixing, and flowability. Percolation segregation requiresdynamic conditions such as those induced by shear and vibration inbulk solids [6], however even small movements can induce segrega-

1 814 863 1031.

B.V.

tion by this mechanism. Given its pervasive nature and impact innumerous applications, percolation segregation in particulate materi-als was the focus of this study. Segregation can be mitigated, if noteliminated, by understanding the factors affecting the mechanisms[11]. Three approaches either to eliminate or to reduce segregationthat have been reported are change of material, change of process, andchange of equipment design [12].

Bridgwater and colleagues were the pioneers in identifying thedominant parameters responsible for segregation [13–20]. Theseresearchers found that the size of particulates is the most dominantparameter contributing towards segregation. Results were laterconfirmed by several researchers (for instance, [21–26]). Subsequentresearchers have reported that size and density are the two mainparameters for segregation [27–30] of components, especially forblended fertilizers. Furthermore, percolation segregation was shownto be affected by shape in addition to size and density [31–33] as wellas the operating conditions [34–35].

Under vibration conditions, the effect of physical properties suchas particle size and distribution, density, and shape has been reported[36–37]. Binarymixtures of glass beads and steel of the same sizewerestudied to observe the effect of vertical vibration on segregation [38].When a mixture of different size particles was vibrated, the largestsize particles rose to the top or sank to the bottom depending uponvibration frequency and amplitude [39]. Rise and sink of an intruder ina granular material bed depend on the particles' properties andintensity of vibrations [40]. Researchers have also studied the rise andsink behavior of an intruder in a vertically vibrated cohesive granular

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Fig. 1. Primary segregation shear cell of generation two (PSSC-II).

Fig. 2. Typical particle size distributions for blended fertilizer major raw ingredients.

74 A.K. Jha, V.M. Puri / Powder Technology 195 (2009) 73–82

material [41]. Effect of shape and density on segregation in binarymixtures was quantified [42]. Limited understanding of time-dependent percolation segregation has been achieved by researchers[24,43–46]. Experiments were conducted for a limited number ofbinary size mixtures of glass beads (ideal material) under two strainrates, when a layer of fines was introduced at the surface of a bed ofcoarse particles [44]. Although wealth of information is available onsegregation in the literature, these results have limited application inthe real-worldmaterials (actual materials used in industry for productmanufacturing) either because the material was ideal material orstudies were time-independent. One of the efficient approaches togaining a deeper understanding of percolation segregation mecha-nism is by studying the cumulative effect of two or more parameters,i.e., allows one to build a roadmap by understanding the influence ofindividual parameters and their interactions that contribute towardoverall segregation. While size has been identified as a dominantparameter, the magnitude (i.e., amount) and rate of percolationsegregation of fines (during shearmotion conditions) are not known apriori for specific operational conditions. Therefore, appropriate testssufficiently representing the operational conditions must be per-formed to determine the amount and rate of segregation of fines.

Based on the above literature review and their limited applicationto real-world materials, the aim of this research was to study time-dependent percolation segregation in binary mixtures of particulatematerials. Accordingly, the specific objectives of this study were todetermine the effect of: 1) strain rate on size ratio 2) coarse size andfines size, and 3) materials used to formulate binary size mixtures.

2. Materials and methods

The second generation primary segregation shear cell (PSSC-II) hasfive main components [22]: shear box, measurement system, sievesystem, drive system, andmain frame (Fig. 1). The details, capabilities,and limitations of the PSSC-II can be foundelsewhere [22,45]. A sieveofopening size 2360 µmwas used throughout the experiments based onpreliminary tests with different binary mixtures to ensure that coarsesize particles did not block the sieve openings, while allowing the finesto fall unhindered through the sieve openings.

Binary, ternary, and quaternary size distributions were made andstudied in addition to continuous distributions at different strain rates.The motivation for binary and multi-size study was to formulatemixtures representative of continuous size distribution when studyingpercolation response under different motion conditions. Binary sizemixture is considered to be the foundation ofmulti-size and continuousmixture study. Therefore, results are presented for binarymixture in thisarticle. Results of multi-size and continuous mixtures will be presentedin subsequent articles. In the present article, nine different binary sizeratios of potash and six different size ratios of urea in different mixingratios were studied. The segregation results were analyzed using

segregation determining metrics such as the effect of size ratio andmixing ratio on segregation, collected segregated fines mass, segrega-tion rate (SR), and normalized segregation rate (NSR). NSR was definedas the amount of fines percolated from the initial fines in a binarymixture for the total time of operation of PSSC-II (kg/kg-h). Strain ratevs. strain is the most common approach to compare data among thescientific community; however, since our process is periodic with nearramp-type strain profile and segregation in bagged fertilizers being ourmotivation, time is the most appropriate representation in this specificsituation. Therefore, analysis and interpretation of segregated finesmass, SR, and NSR are done with respect to time.

2.1. Test material selection and parameter determination

Two different materials, urea and potash, were selected as testmaterials for studying percolation segregation in binarymixtures. Thesetwomaterials are the twomajor raw ingredients used formanufacturingblended fertilizers and selected based on their extreme shape anddensity among four raw ingredients, i.e., urea, potash, phosphate, andfiller. Materials used in this research are referred to as real-worldmaterials because these are used for manufacturing products. End usersor blend manufacturers have very minimal to no control over particlesize, size distribution, and shape as received from raw ingredientdistributors/manufacturers. Elongated particle is a commonly useddescriptor for potash along with angular-shaped and irregular becauseof a large variety of geographical origins of this material.

The particle densities of spherical urea and angular potash were1459 kg/m3 (standard deviation, SD=2 kg/m3) and 2291 kg/m3,(SD=3 kg/m3), respectively. Typical particle size distributions ofsamples of urea and potash collected from a blend plant acrossCommonwealth of Pennsylvania are given in Fig. 2. Herein, potash andurea were representatives of irregular or angular-shaped andspherical-shaped particles, respectively [47]. The mean measured

Table 2Segregation results for binary mixtures with three coarse sizes for potash.⁎

Coarse Size(µm)

Sizeratio

Mixingratio

Strain rate(Hz)

Collectedfines (g)

Segregationrate (kg/h)

NSR(kg-kg/h)

3350–4000 2.4 50:50 0.25 232.50 (5.1) 0.47 (0.01) 0.97 (0.02)0.50 369.5 (6.5) 0.74 (0.01) 1.54 (0.03)1.00 409.35 (4.2) 0.82 (0.01) 2.27 (0.02)

2.0 63:37 0.25 196.22 (6.1) 0.39 (0.01) 0.63 (0.02)0.50 280.2 (5.7) 0.56 (0.01) 0.90 (0.02)1.00 333.3 (4.01) 0.67 (0.03) 1.08 (0.05)

1.7 37:63 0.25 32.9 (4.8) 0.07 (0.01) 0.11 (0.02)0.50 39.0 (7.2) 0.08 (0.01) 0.13 (0.02)1.00 43.6 (3.5) 0.09 (0.01) 0.14 (0.01)

2800–3350 2.0 63:37 0.25 138.6 (9.2) 0.28 (0.02) 0.75 (0.05)0.50 216.6 (5.3) 0.43 (0.01) 1.17 (0.03)1.00 292.2 (7.2) 0.58 (0.01) 1.58 (0.04)

1.7 50:50 0.25 117.8 (5.5) 0.24 (0.01) 0.49 (0.02)0.50 205.3 (4.7) 0.41 (0.05) 0.86 (0.10)1.00 298.2 (8.7) 0.60 (0.01) 1.24 (0.03)

1.4 50:50 0.25 29.3 (1.1) 0.06 (0.00) 0.12 (0.00)0.50 39.5 (1.4) 0.08 (0.00) 0.16 (0.01)1.00 53.3 (3.4) 0.11 (0.01) 0.22 (0.02)

2360–2800 1.7 63:37 0.25 68.7 (1.0) 0.14 (0.00) 0.46 (0.01)0.50 57.7 (6.5) 0.12 (0.01) 0.39 (0.04)1.00 76.2 (8.8) 0.15 (0.02) 0.51 (0.06)

1.4 63:37 0.25 40.9 (0.5) 0.08 (0.00) 0.21 (0.00)0.50 43.8 (1.4) 0.09 (0.00) 0.23 (0.01)1.00 63.0 (3.0) 0.13 (0.01) 0.33 (0.03)

1.2 63:37 0.25 12.9 (1.7) 0.03 (0.00) 0.07 (0.01)0.50 12.8 (0.8) 0.03 (0.00) 0.07 (0.00)1.00 17.4 (0.9) 0.03 (0.00) 0.09 (0.00)

⁎ SD (standard deviation) values in parentheses.

75A.K. Jha, V.M. Puri / Powder Technology 195 (2009) 73–82

sphericity of urea and potash particles of size range 3350–4000 µmwere 0.97 (SD=0.02) and 0.78 (SD=0.08), respectively [48]. In orderto incorporate operating conditions that the fertilizer blends areexpected to experience during handling, strain and strain rate werethe experimental design variables. In the context of this study, strainrate is defined as the cycles of movement (intensity of to-and-fromovement) of the shear box per second. Accordingly, testing wasconducted at strain rates of 1.0, 0.5, and 0.25 Hz [49]. In all percolationsegregation of 15 binary size ratios of urea and potash was quantifiedat strain of 6% and bed depth of 85 mm. Strain, strain rates, and beddepth values were selected based on head space available, motionexperienced under different handling conditions, and fill depth ofbagged blended fertilizer of low analysis such as 10–10–10. Thenumeric 10–10–10 represents the percentage of urea, phosphate, andpotash in this order. In subsequent publications, results for additionalstrain and bed depth will be presented.

Different coarse and fine size ranges of the test material wereobtained using US standard sieve of (2)1/4 series. Potash and ureawere received from local fertilizer blend plant facilities.

Three size ranges (3350–4000, 2800–3350, and 2360–2800 µm)were designated as coarse and three fines size ranges (2000–2360,1700–2000, and 1400–1700 µm) were designated as fines in thepresent study (Table 1). Since size spread of urea was small comparedwith potash, fines in the size range 1400–1700 µm was approximately0.2% of the overall distribution (Fig. 2); therefore, due to time and costconstraints of the project, the fines size of 1550 µmwas not included inthe segregation study of urea, i.e., was deferred to a subsequent study.Size ratio of binary mixture was defined as the ratio of mean size ofcoarse particles tomeansizeoffineparticles. For potash, three size ratiosfor each coarse sizes 3675 µm, 3075 µm, and 2580 µmwere 2.4, 2.0, 1.7and 2.0,1.7, 1.4, and 1.7, 1.4, 1.2, respectively, (Table 1). For urea, two sizeratios for each coarse size 3675 µm, 3075 µm, and 2580 µmwere 2.0,1.7,and 1.7, 1.4, and 1.4, 1.2, respectively. Different mixing ratios were usedfor different size ratios based on weight proportion of different size(Table 1) distributions found in low analysis fertilizer blend samplecollected from blend plants.

2.2. Test condition and experimental design

Coarse size particles were mixed with fines size particles in a 225-Wsix-speedbench-topmixer (Model-106772N, Type-M27,General Electric,Marketed byWal-Mart Stores Inc., Bentonville, AR). Starting condition ofall thematerials was the same. Initial tests showed that 30–35 s at lowest

Table 1Experimental design for binary size mixtures for potash and urea.⁎

Material Strain rate(Hz)

Coarse size(µm)

Fines size(µm)

Sizeratio

Mixingratio

Number

Potash 0.25 3350–4000 1400–1700 2.4 50:50 90.50 1700–2000 2.0 37:631.00 2000–2360 1.7 37:63

Potash 0.25 2800–3350 1400–1700 2.0 63:37 90.50 1700–2000 1.7 50:501.00 2000–2360 1.4 50:50

Potash 0.25 2360–2800 1400–1700 1.7 63:37 90.50 1700–2000 1.4 63:371.00 2000–2360 1.2 63:37

Urea 0.25 3350–4000 1700–2000 2.0 37:63 60.50 2000–2360 1.7 37:631.00

Urea 0.25 2800–3350 1700–2000 1.7 50:50 60.50 2000–2360 1.4 50:501.00

Urea 0.25 2360–2800 1700–2000 1.4 63:37 60.50 2000–2360 1.2 63:371.00

Total (six replications) (sum of numbers in column 7=45×number ofreplications=6)=270

⁎Strain of 6%.

rpmwas sufficient to uniformly mix the binary size samples. In order toevaluate the quality of mixing, materials of binary mixtures were mixedand sampled from mixer bowl by a large spoon. These samples weresieved to determine the proportion of coarse and fines, if the proportionof fines and coarse in each sample was within 95% confidence intervalthen the ingredientswere consideredwellmixed. Aftermixing, mixtureswere gently placed in shear box to avoid any further segregation. Fromstatistical analysis of data, a separate experimental designwas consideredfor both angular and spherical-shaped materials including dissimilaramount of fines (Table 1). Based on published results [43,45,49] andpreliminary testing with fertilizer blends, six replications were done foreach set of experiments for testing percolation segregation using PSSC-II.A complete block design was selected for data analysis. A set of coarseparticles was considered as a block of experiment. Within each block, alltreatments (replicate=1×6=6) were randomly assigned. Of the six

Table 3Segregation results for binary mixtures with three coarse sizes for urea.⁎

Coarse size(µm)

Sizeratio

Mixingratio

Strain rate(Hz)

Collectedfines (g)

Segregationrate (kg/h)

NSR(kg-kg/h)

3350–4000 2.0 37:63 0.25 81.6 (1.8) 0.16 (0.00) 0.36 (0.01)0.50 140.3 (4.0) 0.28 (0.01) 0.62 (0.02)1.00 150.9 (8.7) 0.30 (0.02) 0.67 (0.04)

1.7 37:63 0.25 31.2 (1.7) 0.06 (0.00) 0.14 (0.01)0.50 27.3 (2.8) 0.05 (0.01) 0.12 (0.01)1.00 30.5 (4.2) 0.06 (0.01) 0.14 (0.02)

2800–3350 1.7 50:50 0.25 27.5 (1.3) 0.05 (0.00) 0.16 (0.01)0.50 41.2 (3.7) 0.08 (0.01) 0.24 (0.02)1.00 47.4 (3.3) 0.09 (0.01) 0.27 (0.01)

1.4 50:50 0.25 21.9 (2.3) 0.04 (0.00) 0.13 (0.01)0.50 19.8 (5.8) 0.04 (0.01) 0.11 (0.03)1.00 16.7 (1.5) 0.03 (0.00) 0.10 (0.00)

2360–2800 1.4 63:37 0.25 9.8 (1.0) 0.02 (0.00) 0.07 (0.01)0.50 12.3 (0.7) 0.02 (0.00) 0.09 (0.01)1.00 17.3 (1.9) 0.03 (0.00) 0.12 (0.01)

1.2 63:37 0.25 9.9 (0.5) 0.02 (0.00) 0.07 (0.00)0.50 7.0 (0.4) 0.01 (0.00) 0.05 (0.00)1.00 7.3 (1.0) 0.01 (0.00) 0.05 (0.01)

⁎ SD (standard deviation) values in parentheses.


replicates, three replicates for each set were completed using load cellsand three replicates were completed by collecting segregated fines in apan. Results of segregation determining metrices are given in tabularform based on six replications. However, only three replications wereincluded for the graphical representation of results because of the limitedcapacity of the load cells (b3.5 g), i.e., load cells were not able to collectdata effectively up to 30 min with the desired accuracy for all the sizeratios. The segregated fines mass values were measured and segregationdetermining parameter, normalized segregation rate, was deduced fromthe finesmass values. These results were compared statistically based on95% confidence interval. All tests were conducted in the environment-controlled laboratory with average temperature of 22 °C±3 °C andrelative humidity less than 40%.

3. Results and discussion

The results of segregated fines mass and normalized segregationrate are summarized in two subsections, i.e., the effect of strain rateon size ratio and on materials. The results for potash and urea atthree strain rates 0.25, 0.5, and 1.0 Hz are presented in Tables 2 and 3,respectively.

Fig. 3. Comparison of percent segregated fines for three strain rates when coarse sizepotash was 3675 µm for size ratios (a) 2.4, (b) 2.0, and (c) 1.7.

Fig. 4. Comparison of percent segregated fines for three strain rates ratios of potashwhen coarse mean size was 3075 µm at size ratios (a) 2.0, (b) 1.7, and (c) 1.4.

3.1. Segregated fines mass

Segregated fines mass is the mass of fines exiting from the primarysegregation shear cell screen (placed at the bottom of shear cell),whichwas continuously collected in a pan andweighed and also usingload cells. This is the primary experimental data obtained fromPSSC-II,i.e., is sufficiently important and forms the basis for estimation ofsegregation rate metrics SR and NSR. As expected, with increasingstrain rate from 0.25 Hz to 1.0 Hz, the percent segregated fines massincreased with time for binary mixtures. However, for smaller sizeratios effects of strain rate were small compared to larger size ratios.

3.1.1. Strain rate effect on size ratioFigs. 3 to 5 compare the percent segregated fines of angular-shaped

potash at three strain rates of 1.0, 0.5, and 0.25 Hz for three size ratioseach for the three coarse sizes 3675; 3075; and 2580, respectively. InFig. 3, of the total time, for the first 10 min fines were collected at 30 sinterval and thereafter 120 s interval due to the slow down indischarge of fines. The percent segregated fines mass was highest andlowest at 1.0 and 0.25 Hz strain rates, respectively for binary size ratios2.4, 2.0, and 1.7 of potash. As expected, percent segregated fines mass

Fig. 5. Comparison of percent segregated fines for three strain rates ratios of potashwhen coarse mean size was 2580 µm at size ratios (a) 1.7, (b) 1.4, and (c) 1.2.

Fig. 6. Comparison of percent segregated fines between potash and urea for size ratio2.0 when absolute coarse size was 3675 µm at strain rates (a) 1.0 Hz, (b) 0.5 Hz, and(c) 0.25 Hz.


at 0.5 Hz was in-between percent segregated fines mass at strain rates1.0 Hz and 0.25 Hz. At strain rate 1.0 Hz, after 15 s, 3.8%, 1.8%, and 0.6%of the total initial fines present in the binary mixtures were collectedfor size ratios 2.4, 2.0, and 1.7, respectively. At the end of 30 min, themass segregated at these strain rates were 85.0%, 56.1%, and 7.3%,respectively (Fig. 3) (pb0.05). At strain rate 0.5 Hz, after 15 s, 3.1%,1.6%, and 0.5% of the total initial fines present in the binary mixtureswere collected for size ratios 2.4, 2.0, and 1.7, respectively. At the endof 30 min, the mass segregated at these strain rates were 79.6%, 49.5%,and 7.1%, respectively (Fig. 3) (pb0.05). At strain rate of 0.25 Hz, after15 s, 2.3%, 1.3%, and 0.3% of the total initial fines present in the binarymixtures were collected for size ratios 2.4, 2.0, and 1.7, respectively. Atthe end of 30 min, the mass segregated at these strain rates were47.6%, 32.0%, and 4.7%, respectively (Fig. 3) (pb0.05). The segregatedfines mass for size ratio 1.7 was not significantly different at threelevels of strain rates (pb0.05) [50]. At these three strain rates,however, the percent segregated fines for size ratio 2.0 was closer to

size ratio 2.4 vs. 1.7. This was attributed to the smaller pore sizes in thecoarse particles bed that were not large enough for fines to pass. Theseresults showed that strain rate has minimal or no effect when sizedifference and porosity were small.

Heap formed at the center during the movement of the shear boxand spread towards the left and right sides (Fig. 1). Both length andheight of heap at the center of the shear box (either left to right orright to left movement) were larger and higher for higher size ratiosand dependent on the coarse and fines size combinations. Large sizecoarse particles rise to the top during shear box motion and tumbledalong the heap towards both left and right sides of the shear box.Coarse particles sank to the bottom along the left and right sides of theshear box. Convective current for coarse particles in small size ratios'combinationwas very small, i.e., was almost negligible. It confirms thefact that fines in binary large size ratios segregate by the convectivemechanism, whereas, small size ratio segregate by the diffusivemechanism [43]. Almost symmetric tumbling motion was observedfor all size ratios. The rate of rise of coarse particle was higher forlarger size ratios but for the same size ratio rate of rise was large forsmall fines. These observations were evident from top and glass

Fig. 7. Comparison of percent segregated fines between potash and urea for size ratio 1.7when absolute coarse size was 3075 µm at strain rates (a) 1.0 Hz, (b) 0.5 Hz, and(c) 0.25 Hz.

Fig. 8. Comparison of percent segregated fines betweenpotash and urea for size ratio 1.4when absolute coarse size was 2580 µm at strain rates (a) 1.0 Hz, (b) 0.5 Hz, and(c) 0.25 Hz.


window on the front of shear box. One of the large sides (out of twolarge sides) was hinged to themain frame so it was stationary (locatedon the back). Movement of one side (front side) made dead zonealong the stationary side. Materials trapped in the dead zone werestationary or had negligible motion.

Percolation of fines increased with increase in strain rates from0.25 to 1.0 Hz for all nine different size ratios of potash tested.Convective current carried coarse size particles to the top during shearmotion for size ratios 2.4 and 2.0 when coarse size was 3675 µm.Large-scale convective current carried the large particles to the topwhich allowed the small particles to percolate downwards. At strainrates of 1.0 and 0.5 Hz similar trend was observed for the above-mentioned particles. For size ratios 1.7, 1.4, and 1.2 for coarse sizes3675 µm; 3075 µm; 2580 µm, respectively, had very small differencesin the percent segregated fines at strain rates of 0.25, 0.5, and 1.0 Hz(pb0.5). The rise of coarse particles to the top was governed by thevery weak convective current. It was evident from the open top of theshear box. These results confirmed that size is the dominantparameter responsible for segregation. With increase in size ratiosfor these three coarse sizes, percolation of fines increased but the

percent segregated fines at strain rates of 0.25 and 0.5 Hzwere close toeach other for size ratio 2.0 when coarse sizewas 3075 µm and for sizeratio 1.7 when coarse size was 2580 µm. The presence of large sizefines in size ratio 1.7 of potash needed strong convective forces todislodge the structure of binary size mixtures. Unlike other two smallsize coarse particles, the coarse size particles were large andgenerated strong convective forces at strain rates 0.5 and 1.0 Hz.Non-linear behavior of the percent segregated fines at three differentstrains rates of 0.25, 0.5, and 1.0 Hz suggests that the viscoelasticattributes of potash particles could be an important consideration. Fordiscussion purposes, size is only taken into consideration due to itsdominant nature during particle segregation. The cumulative effect ofphysical and processing parameters such as size and strain rate,respectively, has more dominant effect than the effect of size alone.Cumulative effect results showed that effect increasedwith increase insize ratio.

3.1.2. Comparison between spherical and angular-shaped materialFigs. 6 through 8 compare the percent segregated fines of urea

(spherical-shaped particles) and potash (angular-shaped particles) atstrain rates 0.25, 0.5, and 1.0 Hz for size ratio 2.0 when coarse size was3675 µm; for size ratio 1.7 when coarse size was 3075 µm; and for size


ratio 1.4 when coarse size was 2580 µm, respectively. In the first fewminutes, the percent segregated fines mass for binary mixturesprepared using spherical-shaped urea and angular-shaped potash wasvery close to each other; however, after few minutes, the percentsegregated fines mass increased very rapidly for angular-shapedpotash in comparisonwith the spherical-shaped urea binary mixturesat three strain rates 0.25, 0.5, and 1.0 Hz. More fines were expected inthe case of potash because of angular-shaped particles (static porosityof 51%, i.e., larger void spaces) and higher particle density comparedwith spherical shape urea particles (static porosity of 44% i.e., smallervoid spaces). Since the shape and density are different of urea vs.potash the objective was to quantify the cumulative (i.e., notpartitioned) effect of shape and density segregation of fines of ureaand potash based on respective absolute size and size ratio.Accordingly, comparison of urea and potash fines segregationpotential is justified.

The difference between the percent segregated fines mass of ureaand potash increased with increase in strain rate from 0.25 Hz to1.0 Hz. Time to exceed the percent segregated fines of potash vs. ureadecreased with increase in strain rates from 0.25 Hz to 1.0 Hz.

Fig. 9. Comparison of NSR for three strain rates when mean coarse size of potash was3675 µm at size ratios (a) 2.4, (b) 2.0, and (c) 1.7.

Fig. 10. Comparison of NSR for three strain rates of potash when mean coarse size was3075 µm at size ratios (a) 2.0, (b) 1.7, and (c) 1.4.

Relationship between time and percent segregated fines during shearrate increase was not linear [49].

At these three strain rates, for size ratio 2.0, the percent segregatedfines for coarse size 3075 µm was higher than coarse size 3675 µmduring 30 min of PSSC-II operation. The size of fines (1550 µm) withcoarse size 3075 µm was smaller compared with the size of fines(1850 µm) with coarse size 3675 µm in binary mixtures of size ratio2.0. A plausible reason of the less segregated fines for coarsemean size3675 µm than 3075 µm is because of small void spaces in the binarymixtures and void spaces were not large enough to let fine particles topercolate [23,24]. The difference in the percent segregated fines massfor these two coarse sizes decreased with decreasing strain rate from1.0 Hz to 0.25 Hz. For size ratios larger than 4.0, it was found that thepercent segregated fines mass for larger absolute size coarse particlesis higher [45]. But for size ratio smaller than 2.4, the size of fineparticles was also the determining factor for the percent segregatedfines from a well mixed binary mixtures. These results showed thatthe percent segregated fines, from a well mixed system, is not onlydetermined by the size of coarse particles but also by the size of fineparticles. However, the results obtained for other two size ratios 1.7

Fig. 11. Comparison of NSR for three strain rates when mean coarse size of potash was2580 µm at size ratios (a) 1.7, (b) 1.4, and (c) 1.2.

Fig. 12. Comparison of NSR between potash and urea for size ratio 2.0 when coarse sizewas 3675 µm at strain rates (a) 1.0, (b) 0.5 Hz, and (c) 0.25 Hz.


and 1.4 were significantly different at three strain rates 1.0, 0.5, and0.25 Hz (pb0.05). The percent segregated fines for size ratio 1.2 wasnot significantly different at these three strain rates (pN0.05).

After placing the test materials urea and potash in the shear box,when PSSC-II was set in motion, particle rearrangement took place.Local bridges formed for urea compared to potash were unstable dueto urea's spherical shape, consequently urea fine particles percolatedthrough the pore spaces more easily. Angular-shaped potash formedsomewhat stable bridges during initial particle rearrangement. Onceparticle rearrangement was completed, the cumulative effect of shapeand density with size in the case of potash was more dominant thansize alone in the case of urea. Here gravity was the dominant drivingforce for fines particle percolation. The particle density and bulkporosity of potash were higher than urea, which resulted in higherpercent segregated fines. Similar results were obtained for binarymixtures comprising different coarse sizes and size ratios (pb0.05).

Bridges formed in potash were broken earlier at higher strain ratescompared to lower strain rates. This was attributed to the greaternumber of movements imparted to the binary mixture bed at higherstrain rates. The cumulative effect of size, shape, and density wasmoredominant for large size ratios compared to small size ratios than size

alone. The effect of shape and density was negligible at smaller sizeratios that led to higher percent of segregated fines in the case of ureacompared to potash. For smaller size ratio 1.4, the effect of strain ratewas not significant (pb0.5).

3.2. Normalized segregation rate

SR and NSR results are similar at strain rates 1.0, 0.5, and 0.25 Hzwhen comparing results for the same mixing ratio; however, differentmixing ratios for some treatments lead to differing results [24]. Themixing ratios for tested binary size mixtures were kept constant fortesting segregation potential of fines at the three mentioned strainrates. Herein, only the NSR results are presented.

3.2.1. Strain rate effect on size ratioFigs. 9 through 11 compare the NSR at three strain rates 1.0, 0.5,

and 0.25 Hz, for three size ratios each 2.4, 2.0, and 1.7 when coarse sizewas 3675 µm; 2.0, 1.7, and 1.4 when coarse size was 3075 µm; and 1.7,1.4, and 1.2 when coarse size was 2580 µm, respectively. Withdecrease in size ratio for all three coarse sizes the NSR decreased withdecrease in strain rate and also with time. When NSR results fordifferent size ratios of potash were compared it was found that size isthe most dominant parameter contributing towards segregation. Size


ratios 2.4, 2.0, and 1.7 when coarse size was 3675 µm (Fig. 9) showedminimal or no difference in NSR at strain rate 0.25 Hz and similarresults were observed for size ratios 2.0, 1.7, and 1.4 when coarse sizewas 3075 µm (Fig. 10) at strain rates 0.25 and 1.0 Hz and size ratios 1.7,1.4 and 1.2 when coarse size was 2580 µm (Fig. 11) at strain rates 0.25,0.5, and 1.0 Hz. The reason of having minimal or no effect of strain rateon NSR is because of the small size difference in coarse and fines andsmaller void spaces. NSR results were significantly different (pb0.05)for three size ratios 2.4, 2.0 and 1.7 at strain rates 1.0 and 0.5 Hz whencoarse size was 3675 µm. The differences were significant (pb0.05)for coarse size of 3075 µm at strain rates 1.0 and 0.5 Hz. Large errorbars in Fig. 10a and b showed larger variability in the NSR at strainrates 1.0 and 0.5 Hz.

3.2.2. Comparison between angular and spherical-shaped materialFigs. 12 through 14 compare NSR of urea and potash at three strain

rates 0.25, 0.5, and 1.0 Hz for size ratios 2.0,1.7, and 1.4, respectively. Asmentioned previously, the large decrease in NSR compared withpotash can be attributed to urea's spherical shape and lower bulkporosity vs. potash binary mixtures that comprised of angularparticles that form higher bulk porosity assembly [49]. Clearly, sizesegregation in conjunction with other physical properties such asshape and density has greater detrimental effect than size alone. Thedifferences in percent segregated fines of urea and potash weresignificant (pb0.05) when fines size 1850 µm was used with three



coarse mean sizes for size ratios 2.0, 1.7, and 1.4. The percentsegregated fines mass difference between urea and potash hasminimal or no effect when fine size 2180 µm was used with threedifferent coarse sizes for size ratios 1.7, 1.4, and 1.2 and were notsignificantly different (pb0.05) at strain rates 1.0 Hz to 0.25 Hz (Tables2 and 3). At three strain rates, size ratio 2.0 was selected to show theeffect of coarse (3675 µm vs. 3075 µm) and fines (1850 µm vs.1550 µm) sizes on the NSR. The NSR of coarse size 3075 µmwas higherthan the NSR of coarse sizes 3675 µm at all three strain rates. Inaddition to fines size difference, the proportion of fines for 3075 µmwas small compared to 3675 µm combinations, i.e., mixing ratios were67:33 and 33:67, respectively (pb0.05). The results obtained for theother two size ratios 1.7 and 1.4 were significantly different at threestrain rates 0.25, 0.5, and 1.0 Hz (pb0.05).

4. Conclusions

Percolation segregation has been extensively studied for materialproperties and processing parameters. In the literature, size has beenthe most studied due to its dominant effect on particle segregation,while shape and density were documented to have secondary effect.There is a paucity of results in public domain relative to the cumulativeeffect of two or more material properties and/or processingparameters on particulate segregation.


Percolation of fines during shear motion decreased with decrease insize ratio for both potash and urea. Percolation segregation of fines wasdependenton size, shape, anddensity. The effect of strain ratewas foundto be more dominant on large size particles compared to small sizeparticles. For small size particles, the effect of strain rate, shape, anddensity were minimal or negligible. It was found that the cumulativeeffect of size and strain rate increased with increase in size ratio.Additionally, the cumulative effect of size, shape, and density has moredominant effect than size alone. The effect of strain rate on percolationsegregation of fines of urea and potash was also dependent on theabsolute coarse and fines sizes for a given size ratio.

Convective current induced large particles to rise to the topproviding pathways for more fines to percolate. Magnitude ofconvective force for coarse size particles was stronger, whichdiminished with decrease in size ratios. The percolation segregationof fines for small size ratios was governed by diffusive mechanismunlike the convective mechanism for large size ratios.

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